There is a continuing need to block the ultraviolet (UV) transmission (wavelength <380 nm) from solar radiation through window glass. In this regard, U.S. Pat. No. 6,106,605 to Basil et al (the entire content of which is expressly incorporated hereinto by reference) discloses a silica-free, abrasion resistant coating comprised of an organic UV absorbing compound, such as hydroxybenzophenone in an inorganic oxide matrix formed by the hydrolysis and condensation of an organo-alkoxysilane. Improvements to UV coatings and their methods of manufacture are still sought.
U.S. Pat. No. 4,051,161 to Proskow (the entire content of which is incorporated hereinto by reference) discloses several alkoxysilane-benzophenone monomers prepared by reacting an alkoxysilane that contains an epoxide moiety with a hydroxyl group on a hydroxybenzophenone to form an α, β hydroxy-ether which covalently links the hydroxybenzophenone and the silane. Thus, according to Proskow '161, monomeric species may be incorporated into polymers containing polysilicic acid (e.g. silica) and hydroxylated fluorocopolymer systems.
UV-absorbing coatings have also been made and sold more than one year prior to the date of the present application in which an n-propanol (alcoholic) solution of tetrahydroxybenzophenone, 3-glycidoxypropyl trimethoxy silane, and acetic acid was applied onto glass as a coating after first partially hydrolyzing the 3-glycidoxypropyl trimethoxysilane component. The coated glass substrate was then heated to between about 200° C. to about 220° C. so as to cross-link the 3-glycidoxypropyl trimethoxysilane component via ring-opening polymerization and cross-linkage thereof. The cross-linked 3-glycidoxypropyl trimethoxysilane moieties therefore serve as a matrix in which the tetrahydroxybenzophenone compound is physically bound.
While the prior technique described immediately above does in fact produce satisfactory UV-absorbing coatings, further improvements are still desirable. For example, it would be highly desirable if the UV-absorbing compound (e.g., tetrahydroxybenzophenone) were bound chemically to the cross-linked 3-glycidoxypropyl trimethoxysilane moieties so as to provide more durable, leach resistant coatings with UV-absorption capabilities. In addition, it would be desirable to effect cross-linkage of the 3-glycidoxypropyl trimethoxysilane moieties at temperatures less than about 200° C. so that production line speeds could be increased thereby improving productivity. Likewise, reduction in production line temperatures will reduce wear and tear of the production line components resulting in longer component life. Finally, prepolymerization and temperature reductions will reduce the amounts of 3-glycidoxypropyl trimethoxysilane lost as volatiles during the coating process making this a less polluting and less costly process. It is towards fulfilling these desirable objects that the present invention is directed.
Broadly, the present invention is embodied in methods of forming UV-absorbent transparent coatings and transparent substrates coated with the same which allow for relatively lower temperature cross-linkage reactions between a UV-absorbent compound and an epoxy alkoxysilane. More specifically, in especially preferred forms of the invention, UV-absorbent coatings on transparent substrates are formed by prepolymerizing a mixture consisting essentially of a hydroxy-benzophenone, an epoxyalkoxysilane and an organic catalyst at an elevated temperature of between about 40° C. to about 130° C. and for a time sufficient such that between about 30 to about 70% of the epoxyalkoxysilane moieties form ring-opened oligomers and polymers with degrees of polymerization of between about 2 to about 2000, and more preferably between about 2 to 200. Such prepolymerized mixture may then be coated onto the surface of a transparent substrate.
Most preferably, the prepolymerized mixture is hydrolyzed prior to being coated onto the substrate in an alcoholic acidic solution.
Preferred for use in the present invention as a UV-absorbent compound is tetrahydroxybenzophenone. The preferred epoxyalkoxysilane is 3-glycidoxypropyl trimethoxysilane (sometimes hereinafter referenced more simply as “glymo”). It is especially preferred that prepolymerization be effected in the presence of a tertiary amine such as triethylamine (TEA) as the organic catalyst which is only one of many possible amine catalysts that one practiced in the art will recognize. In addition, it is also possible to use a basic alkoxide, ROM where M is an alkali metal or alkaline earth metal and RO is any suitable, soluble organic that will react with the glymo epoxy ring or with the RSi(OR)3 group. Likewise, it is possible to use species such as R4NOH and R4POH as catalysts for the ring-opening oligomerization or polymerization of the epoxy group on glymo.
These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.